They couldn't because it had a slope to it.
Embattled Secret Service Director Kimberly Cheatle acknowledged the “buck stops with me” as she responded to criticism of her agency’s disastrous handling of former President Dona…
nypost.com
I guess the sloped roof was a safety concern for highly trained federal agents of the secret service, but it wasn't for the shooter.
I guess the sloped roof had to be avoided so agents wouldn't get hurt, but trump definitely got hurt.
And I see a dozen Mexicans running up and down very steep roofs all day long reroofing houses and they seem fine.